The present invention generally relates to gas sensors, and more particularly to a gas sensor for sensing gas existing within the atmosphere.
Gas sensors which use a metal oxide semiconductor as a gas sensitive material are known. In such gas sensors, a heater layer is provided on a back side of a metal oxide semiconductor layer via electrodes and an insulator layer or, an electrode and a heater coil which also functions as an electrode are provided within a metal oxide semiconductor. The gas sensor senses the gas using the fact that the resistance of the metal oxide semiconductor heated by the heater layer or the heater coil changes depending on the gas adhesion at the surface of the gas sensor.
FIGS. 1A and 1B respectively show a cross section and a perspective view of an example of a thin film gas sensor. A heater layer 2 is formed on a heat resistant substrate 1, and an insulator layer 3 is formed on the heater layer 2. Electrodes 41 and 42 and a gas sensitive layer 51 are formed on the insulator layer 3. The gas sensitive layer 51 partially overlaps the electrodes 41 and 42. An insulator layer (not shown) is provided between the substrate 1 and the heater layer 2 if the substrate 1 is made of a conductive material. Power supply lines 61 and 62 for supplying power to the heater layer 2 connect to the heater layer 2. An output signal from the gas sensitive layer 51 is obtained via lead wires 71 and 72.
FIG. 2 shows a cross section of another example of a conventional thin film gas sensor. In FIG. 2, those parts which are the same as those corresponding parts in FIGS. 1A and 1B are designated by the same reference numerals, and a description thereof will be omitted. In FIG. 2, the electrodes 41 and 42 respectively overlap parts of the gas sensitive layer 51.
On the other hand, FIG. 3 shows a perspective view of still another example of a conventional gas sensor. Heater coils 43 and 44 also function as a pair of electrodes, and a sintered body (gas sensitive material) 52 made of a metal oxide semiconductor is supported between the heater coils 43 and 44. For example, the gas sensitive material 52 has a square shape in which each side is 2 to 3 mm long. Lead wires (not shown) can be connected to the heater coils (electrodes) 43 and 44 so as to obtain an output signal of the gas sensitive material 52. In addition, the heater coils 43 and 44 are embedded within the gas sensitive material 52. This gas sensor also includes a base 12 and electrode pins 8.
However, the gas sensor of the type shown in FIG. 3 has a large power consumption, and the response characteristic is poor because of the large thermal capacity. On the other hand, both the power consumption and response characteristic of the thin film gas sensors shown in FIGS. 1 and 2 are satisfactory, but the sensor characteristics greatly change with time. For this reason, the thin film gas sensors are not suited for practical use.
It may be regarded that the sensor characteristic of the thin film gas sensor deteriorates with time because the size of the crystal grains of the metal oxide semiconductor increases with time. Normally, the gas sensor is used at relatively high temperatures of 300.degree. to 450.degree. C. Hence, when the gas sensor is used under such a relatively high temperature for a long period of time, it may be regarded that the size of the crystal grains of the metal oxide semiconductor increases and the gas adhesion area decreases or the chemical activity decreases, thereby deteriorating the gas sensitivity.